A Thai identification portrait of a 26-year-old female suspect is shown in this Thai Royal Police handout released August 31, 2015. Police probing Thailand's deadliest bombing issued arrest warrants on Monday for two suspects after a raid on a suburban apartment block uncovered possible bomb-making materials. Police were hunting for the 26-year-old Thai woman and a foreign man in his 40s after a weekend search on a property in the Min Buri district uncovered fertiliser, digital watches and an explosives detonator, police spokesman Prawut Thawornsiri said on TV.

Thai police said Monday that they had identified two new suspects — a Thai woman and a man of unknown nationality — wanted in connection with the Aug. 17 shrine bombing in Bangkok that killed 20 people.

Over the weekend police arrested another suspect of unknown nationality and seized bomb-making supplies from two Bangkok apartments, providing incremental clues about an attack whose motivation remains almost entirely opaque.

The two new suspects are a Thai woman — named as Wanna Suansant — and a man of unknown name or nationality: his police sketch shows with short brown hair and a thin mustache, according to the Bangkok Post. Both are thought to have been residents of the second apartment police raided over the weekend, where police discovered the bomb-making materials they say may have been for a second attack.

Among the contents of the apartment, according to police spokesman Prawuth Thavornsiri, were "important bomb-making materials such as gunpowder, urea-based fertilizer which can be used as explosive powder when mixed with other substances, a remote-controlled car with its controller which can be used as a detonator, nuts and bolts, small light bulbs and digital watches,” reports the Associated Press.

That raid came after police arrested their first suspect Saturday at another Bangkok apartment, where they also seized detonators, ball-bearings, and other possible bomb-making supplies. He was found carrying a Turkish passport — believed to be fake — bearing the name Adem Karadag, according to The Telegraph. Authorities have said little about that man’s suspected role in the bombing; Mr. Prawuth only said authorities believed he was “part of the network” that carried out the bombing, reports the Associated Press.

On Sunday, Prawuth said police were working with "a number of embassies" and interpreters to try to establish the man's nationality, adding that he did not speak Thai but spoke some English.

Speculation has grown that the suspect might be part of a group seeking to avenge Thailand's forced repatriation of ethnic Uighurs to China after images circulated online of a fake Turkish passport found in his apartment bearing his apparent picture.

Uighurs are related to Turks, and Turkey is home to a large Uighur community.

Authorities have dodged questions about whether the suspect is believed to be Turkish, saying that he was traveling on a fake passport.

A fourth suspect — a bearded man in a yellow shirt thought to have left the bombs at the scene of the attack — also remains at large, with no further speculation from authorities as to his nationality or possible motives. He was captured on CCTV just before the explosion at the shrine.

Meanwhile, Thai police chief Somyot Pumpanmuang praised the work of his force Monday, rebutting international criticism that they have handled the investigation ineffectively and provided contradictory information to the public and the media. He announced that he intended to give his police officers an $83,000 reward set aside for members of the public who provided tip-offs in the bombing investigation.

"The accomplished work that led to the arrest is truly the work of the authorities and their investigative abilities," he said at a news conference, according to Reuters. "This was the work of the Thai authorities, there were no tip-offs."

Police reported that they tracked down the three suspects after searching through tens of thousands of phone signals near the blast site and narrowing them down to three Turkish numbers that had activated international roaming services, reports the Telegraph.